A DEVICE FOR CENTERING A ROTATIONAL UNIT

Abstract: A device for centering a rotational unit (1) on a rotatable hub (2) for rotation of the unit together with the hub. A centering surface (9) which extends around the circumference of the hub extends essentially from a mounting edge (10, 11) for the rotational unit to a stop surface (8) which determines the axial position of the rotational unit. The rotational unit presents in a corresponding way a centering surface (7) which in its one end transcends into a stop surface (6), which determines the radial position and which is intended for contact with the stop surface of the hub. One of the centering surfaces (7, 9) is divided into a first centering section (12) with a certain larger clearance to the second centering surface (9, 7) and a second centering section (13) with a certain smaller clearance to said second centering surface. The first centering section extends across the main part of the axial distance (b) from the stop surface (8) of the hub to its mounting edge (10, 11). The second centering section extends across a minor section (a) of the axial distance from the stop surface to the mounting edge of the hub.

Description: TITLE: A DEVICE FOR CENTERING A ROTATIONAL UNIT TECHNICAL FIELD: The present invention relates to a device for centering a rotational unit, in accordance with the preamble of appended claim 1. BACKGROUND OF THE INVENTION: In many applications it is of great importance that the rotating units are properly centered in relation to their axes of rotation. Such rotational units, which are mounted on a rotary hub via mounting surfaces, normally present centering surfaces, shaped as cylinder mantles, which are centered against corresponding centering surfaces, shaped as cylinders mantles, on the hub. Since, for production reasons, a certain tolerance range for the centering surfaces of both the rotational unit and the hub exists, the centering surfaces must be dimensioned in such a way that a certain minimum permitted clearance is assured. In certain cases, the clearance may become so large that, to a great extent, the rotational unit becomes excentrically mounted on the hub and that there is an increased risk for it becoming stuck due to rust, which makes its removal more difficult. Applications in which excentrically mounted rotational units may involve functional problems are for example brake drums for motor vehicles. An excentrically mounted brake drum results in a varying braking torque, which in turn leads to vibrations in the steering wheel and/or the vehicle passenger compartment, which normally is perceived as very annoying by the driver. SUMMARY OF THE INVENTION:. The object of the present invention is to eliminate the above-mentioned problems regarding rotational units by means of a satisfactory centering, while maintaining favourable qualities for mounting and a reduced risk for it becoming stuck due to rust (fitting rust) . Said object is achieved by means of a device according to the present invention, the characterizing features of which will become apparent from the subsequent claim 1. BRIEF DESCRIPTION OF THE DRAWINGS: The invention will now be described in greater detail with an embodiment, with reference to the annexed drawings, in which Fig. 1 shows an axial cross-section of a rotational unit in the form of a brake drum mounted on a hub, according to the present invention, Fig. 2 shows, on a larger scale, a section of the brake drum and the hub, Fig. 3 shows, on a further enlarged scale, a section of the brake drum and the hub clearly showing their mutual mounting surfaces, Fig. 4 shows the brake drum and the hub during a first mounting stage, and Fig. 5 shows an end view of the brake drum. PREFERRED EMBODIMENTS: The chosen example shows a rotational unit in the form. of a brake drum 1 which is mounted on a hub 2 which is rotatable around a geometric axis 3 of rotation. The hub may be fixed to a not shown drive shaft of a motor vehicle, the brake drum 1 being adapted to be rotatably fixed to the hub and thus adapted to rotate together therewith, and thus also together with the drive shaft. The brake drum is normally arranged on the inside of a vehicle wheel and adapted to rotate together with the wheel. The brake drum forms part of a conventional brake system with not shown brake shoes which are carried by the vehicle so that they essentially do not rotate with the brake drum but are adapted to brake the vehicle by frictional contact against the cylindrical brake surface 4 of the brake drum, the centricity of which in relation to the axis 3 of rotation is of great importance for the braking function and comfort in the vehicle. The brake drum is a separate rotational unit with a rear section 5 which extends essentially in a radial plane and which presents mounting surfaces 6, 7 intended for cooperation with corresponding mounting surfaces 8, 9 on the hub 2. Fig. 2 shows a slightly enlarged partial view of Fig. 1, which is chosen so that it is apparent which section of the brake drum and the hub forms the mounting surfaces 6-9. The mounting surfaces form guiding surfaces which secure the position of the brake drum, partly for centering, i.e. a guiding in a radial direction, and also for guiding or securing the position in an axial direction. Also, the axis of symmetry and the axis 3 of rotation of the drum should coincide as far as possible, i.e. they should not present any relative angle. The guiding in the axial direction is assured by the first mounting surface 8 of the hub 2, which extends in a radial plane and forms a stop surface for the corresponding mounting surface 6 of the drum 1. The guiding in the radial direction is assured by the centering surface 9 of the hub, which is shaped essentially as a cylinder mantle and which is radially outwardly facing (convex) , and also by the centering surface 7 of the drum which is also shaped essentially as a cylinder mantle and is radially inwardly facing (concave) towards the axis 3 of rotation. The shaping of the mounting surfaces appears in greater detail in Figs. 3 and 5. From these figures it is apparent that the cylindrical centering surface 9 on the hub extends, as seen in the axial direction, essentially between the stop surface 8 of the hub and the free edge 10 of the hub which presents a sloping section 11 which is shaped as the mantle of a cone in order to facilitate the mounting of the drum 1. The centering surface 7 of the drum, which surface is shaped as a cylinder mantle, is divided according to the invention into two centering sections 12, 13, of which a first centering section 12 is located next to the stop surface 6 of the drum and extends over the main part of the entire centering surface of the drum, i.e. the surface which cooperates with the centering surface 9 of the hub. The first centering section 12, which is shaped essentially as a cylinder mantle, is arranged at a radial distance from the axis 3 of rotation of the drum such that this centering section presents a relatively larger clearance with respect to the centering surface 9 of the hub 2. The second centering section 13 is positioned beyond the first centering section, as seen from the stop surface 6, and is positioned in relation to the first centering section 13 at a somewhat smaller radial distance from the axis 3 of rotation of the drum. As is apparent from Fig. 5 and according to a preferred embodiment, the centering section 13 is divided into four partial surfaces 17, 18, 19 and 20, each of which only occupying 10-15° of the mantle surface of an entire cylinder. According to the invention, the second centering section occupies a shorter distance a of the active width b of the centering surfaces than the first centering section, for example less than a fifth, more precisely for example a tenth of the total width b of the centering surface 9 of the hub. In absolute dimensions, for example the width a of the second centering section 13 may be approximately 1 mm, whereas the width b of the centering surface 9 of the hub may be approximately 10 mm. The clearance between the drum 1 and the hub 2 over the first centering section 12 may for example be Δj = +0,056 → +0,218 mm, whereas the corresponding clearance over said second centering section 13 may be for example Δ 2 = -0,08 → +0,06 mm. With the proposed clearance, it has been verified that the radial distorsion has been reduced by approximately 0,05 to approximately 0,1 mm. Due to the fact that the width a is chosen so small, the clearance may thus even be negative before the mounting, thereby involving a plastic deformation of the active centering surfaces. During mounting, a deformation of said first and/or second centering section 12, 13 occurs, whereby essentially a zero clearance is obtained, i.e. a full contact between the centering sections 12, 13. The section which is deformed, or deformed the most, depends mainly on the difference in hardness between the centering sections. By intermittently dividing the centering section 13, as is shown in Fig. 5, this deformation is facilitated. Between the two centering sections 12, 13 there is a transition surface in the form of a sloping surface 14, which has the shape of a cone mantle and which, due to its shape provides a successive transition between the centering sections and thereby also an easier mounting. With the design of the mounting surfaces 6, 7, 8, 9 of the brake drum 1 and the hub 2, respectively, as described above, a considerably improved centering of the brake drum 1 in relation to the axis 3 of rotation of the hub 2 is thus obtained, without making the mounting of the brake drum more difficult. The manner in which the brake drum is mounted on the hub will now be described with reference mainly to Fig. 4. The brake drum 1 is mounted on the hub 1 with an essentially axial lateral movement in the direction of the arrow 15, with assistance firstly from both the sloping edge surface 16 of the drum and the sloping edge surface 11 of the hub, in order to achieve a guiding towards the centering surfaces. Thereafter, the centering section 12 provides a first approximate centering of the drum brake in a first mounting stage, which is shown in Fig. 4. In practice, as is also shown in Fig. 4, the brake drum 1 is not maintained in an entirely correct centered manner in relation to the hub 2, but the first centering section 12 will, at least along some section of its circumference, be in contact with the centering surface 9 of the hub, thus providing an approximate centering of the drum during the main part of the mounting motion of the drum on the hub. In a second and final mounting stage which ends with the stationary mounted condition of the drum which is shown in Fig. 3, a final centering of the drum 1 in relation to the hub 2 is carried out by mounting the second centering section 13 with its smaller clearance on the front section of the centering surface 9 of the hub. In practice, in most cases the drum is mounted by pressing, due to the narrow or even negative tolerances. The press mounting is preferably carried out by tightening screws in screw joints which extend through the drum and the mounting surfaces 6, 8. With negative tolerances, the above- mentioned deformation takes place in the edge and centering surfaces 9, 11, 13, 14, which are pressed towards each other. The guiding to this position is assured by means of the sloping surface 14 which thus forms a transition between the first and the second centering section. A rotationally fixed fastening of the drum to the hub is usually assured by means of the above-mentioned screw joint. For production reasons, the screw joints themselves can not satisfy the demand for centering of the drum. The invention is not limited to the embodiments mentioned above and shown in the drawings, but may be varied within the scope of the appended claims. For example, the rotational unit 1 may be something else besides a brake drum. Furthermore, the sloping 14 may have another, for example a rounded, cross-sectional line. The surfaces which are shaped as cylinder mantles need not be shaped continously as cylinder mantles, as seen in a radial plane, but may be formed as splines. Furthermore, the mounting surfaces 6, 8 do not have to be continous but may be profiled in order to provide a rotationally fixed form engagement between the drum and the hub. The sloping and thus also the smaller centering section may be arranged in the radially inner part, in the shown example the hub, and is thereby placed closest to the stop surface of the hub, that is, inside of the larger centering section with a greater clearance. The hub is assumed to be the unit which is centrically rotatable around the axis 3 of rotation. However, this does not have to be the radially inner part with a radially outwardly facing centering surface, but may instead be the radially outer unit with a radially inwardly facing centering surface, whereas the rotational unit constitutes the radially inner unit with a radially outwardly facing centering surface. The second centering section 13 may consist of three or more smaller partial surfaces, alternatively an entire cylindrical surface or two larger partial surfaces. In embodiments with one centering section 13 which consists of smaller individual partial surfaces, the distance a may be chosen greater than in the case with one single surface. The active area, the tendency to be deformed and the clearance are, together with the coefficient of friction of the surfaces, important parameters which influence the force ' needed for the mounting.

Claims: CLAIMS: 1. Device for centering a rotational unit (1) on a rotatable hub (2) , for rotation of the unit together with and being mounted on the hub, the hub as well as the rotational unit comprising mounting surfaces (6-9) for the guiding of the position of the rotational unit both axially and radially, which mounting surfaces (8, 9) on the hub comprise a centering surface (9) which extends around the circumference of the hub and which extends essentially from a mounting edge (10, 11) of the rotational unit to a stop surface (8) which determines the axial position of the rotational unit, the rotational unit presenting in a corresponding way a centering surface (7) which at its one end transcends into a stop surface (6) which determines the radial position and which provides a fixed contact against the stop surface of the hub in a mounted condition, c h a r a c t e r i z e d i n that one of the centering surfaces (7, 9) is divided into a first centering section (12) with a certain larger clearance to the second centering surface (9, 7) and a second centering section (13) with a certain smaller clearance to said second centering surface, that the first centering section extends across the main part of the axial distance (b) from the stop surface (8) of the hub to its mounting edge (10, 11) , that the second centering section extends across a smaller section (a) of the axial distance from the stop surface to the mounting edge of the hub, that the centering surface (9) of the hub (2) is radially outwardly facing, that the centering surface (7) of the rotational unit (1) is radially inwardly facing, and that said first centering section (12) is located closest to the stop surface (6) of the rotational unit (1) and that said second centering section (13) is located closer,,to the mounting edge (10, 11). 2. Device according to claim 1, c h a r a c t e ¬ r i z e d i n that the centering surfaces (7, 9) are essentially shaped as cylinder mantles. 3. Device according to claim 1, c h a r a c t e - r i z e d i n that the transition between the centering surfaces (7, 9) is a sloping surface (14). 4. Device according to claim 1, c h a r a c t e ¬ r i z e d i n that said smaller section (a) is less than a fifth of the axial distance (b) . 5. Device according to claim 1, c h a r a c t e ¬ r i z e d i n that said second centering section (13) is divided into several partial surfaces (17-20) . 6. Device according to claim 5, c h a r a c t e ¬ r i z e d i n that the partial surfaces (17-20) occupies only a minor section of the total circumference. 7. Device according to claim 6, c h a r a c t e ¬ r i z e d i n that the partial surfaces are at least three in number. 8. Device according to claim 6, c h a r a c t e - r i z e d i n that the partial surfaces (17-20) are symmetrically distributed along the circumference. 9. Device according to claim 6, c h a r a c t e ¬ r i z e d i n that each of the partial surfaces (17-20) occupies approximately 10-15° of the entire circumference. 10. Device according to any of the preceding claims, c h a r a c t e r i z e d i n that said smaller clearance is negative before the mounting, and that said first and/or second centering section (12, 13) is adapted to be deformed essentially to a zero clearance during mounting.

Inventors: FAGERGREN MATS, OHLSON ROLF, FAGERGREN, MATS, OHLSON, ROLF
Applicants: FAGERGREN MATS, OHLSON ROLF, VOLVO AB, AB VOLVO, FAGERGREN, MATS, OHLSON, ROLF
Application year: 1994
Main classification: F16D1
IPC codes: F16D1, F16D65